1. Log in to the Google Admin console with a user which has sufficient permissions to create and assign whatever admin role you require for the new user.

2. Add the free-tier of the “Cloud Identity” service using [fly-out side menu] > Billing > Get more services, choose Cloud Identity in the left column, then Cloud Identity Free. The free “Cloud Identity” service will be added to every user. Depending on the license assignment configuration for domain.com, the Admin console may offer help to disable automatic-licenses (which will matter for the new user you are about to create, as you do not want it to receive any licenses for paid services). There is information about automatic licensing here.

3. Go to Billing > License settings – Make sure Google Workspace Automatic licensing is OFF

4. Create new user.

Referenced: https://webapps.stackexchange.com/questions/100428/how-do-i-add-an-external-administrator-to-a-google-suite